This past weekend's 'Barn Door' CLE' saw a lot of participation, in spite of the mid-summer doldrums and the universal nasty lightning noise.
After reading CLE-organizer Brian Keyte's interesting posting to the ndblist, describing his homebrew single transistor regenerative receiver, I was inspired enough to dig out the soldering iron and build one for myself.
The circuit widely described as a '1AD regen' was originally designed several years ago by crystal-radio guru Mike Tuggle out in Hawaii. Mike is widely known for his exquisitely designed "Lyonodyne" DX crystal radio, which inspired an entire decade of intense DX crystal radio building activity back in the late 90's.
|Mike Tuggle's 'Lyonodyne' DX Crystal Radio
His '1 Active Device' medium-wave regen consists of a handful of simple components and a MOSFET that is up to the task. His design was, and still is, being duplicated by many throughout the NDB DX listening community.
|Mike's original 1AD regen
Mike recently sent me the circuit diagram that he eventually settled with, showing two possible ways of extracting audio with the widely popular Bogen T725 output transformer.
My own version used Mike's output scheme on the right and the RF circuitry in Roelof Bakker and Steve Ratzlaff's modified version, shown below. I had to increase the tickler (feedback) winding from 4 turns to 11 turns in order to get any regeneration. With 11 turns, my regeneration kicked-in at about three-quarters from the end of the regen control. At some point in time, while testing, some of the leads from my Bogen T725 output transformer momentarily brushed against each other, smoking my FET. Upon replacing it with another BF966, the receiver was much hotter, with regeneration kicking-in very close to the start of the pot. I'm not sure if the first FET was already damaged or that there is enough variation from FET to FET (of the same type) to make some have more gain than others. Accidentally blowing up the FET proved beneficial in the end!
Although most users employ sensitive sound powered headphones with their regens, including myself, some use modern phones with an extra stage of audio, such as an LM386.
Although not nearly as pretty as Mike's regen, my own 1AD was built in an afternoon, just for the recent Barn Door CLE.
|My own MW 1AD regen
I later added a calibrated dial plate, breaking the segment from 200kHz to 550kHz into two separate bands, with calibration ~ +/- 1kHz so that I pretty much new where I was tuning at all times. This allowed me to target specific beacons and wait for them to fade up.
When first published, Mike suggested the BF966 MOSFET seemed to work well and I purchased five of them with the intention of someday building a 1AD for the NDB band. The purchase came in handy, as today, the BF966 is no longer available. Apparently the similar, and still available BF998 works well (from tests done by Steve Ratzlaff) but is now in an SMD package at just 18 cents! No doubt there are dozens of MOSFETS that will do the job and at these prices, experimenting with various devices would be a very worthwhile project.
When operating, the regen proved to be surprisingly sensitive, and by using my 10' x 20' loop, I was able to put my local pest (AP-378 kHz and 1/2 mile away) into a deep null so that its huge signal was no longer blocking the top half of the NDB band. The null allowed me to hear beacons within a few kHz of the blowtorch signal as shown below:
I ended up with 70 stations logged, including a couple of Alaskans, in spite of the horrendous lightning noise on all three nights. Doing another 'Barn Door CLE' in the middle of the quiet DX season would be much more exciting and several 1AD users have indicated an appetite for such an event. If you put something together please let me know as having an army of 1AD's ready to go would be a great incentive to schedule another Barn Door weekend!